Programming Forums > Application Development > C#

Arla · Mar 18th, 2008, 2:29 PM

So I was playing around with the scrabble problem that was posted, but playing in C# and ran into a quandry, that I'm not 100% sure how to get out of

I have a recursion routine to figure out all the possible permutations of an array of int's as follows

(Toggle Plain Text)

		public static ArrayList permute(int[] v,int start, int n)
		{
			ArrayList data = new ArrayList();
			if (start == n-1)
			{
				for (int i=0;i<v.Length;i++)
					System.Console.Write("{0} ",v[i]);
				System.Console.WriteLine();	
				data.Add(v);
			}
			else
			{
				for (int i= start; i<n; i++)
				{
					ArrayList back = new ArrayList();
					int tmp = v[i];
					v[i]=v[start];
					v[start]=tmp;
					back = permute(v,start+1,n);
					foreach(object fromPermute in back)
						data.Add(fromPermute);
					v[start] = v[i];
					v[i]=tmp;
				}
			}
			return data;
		}

The problem is that when it does the data.Add(v) it's not really adding v to the data ArrayList, it's just adding a pointer to v to the ArrayList, as such when v gets updated, every pointer to v is pointing to a new version of it, and as such while this returns the right number of records, they are all the same.

Is there any easy way I should be getting around this? Playing around with it some I know I CAN get around it but making it a comma delimited string, but that's not exactly elegant.

Arla · Mar 18th, 2008, 2:43 PM

See and I answered my own question.

Changing the line from data.Add(v) to data.Add(v.Clone()) resolved the issue, as the return isn't v, it's a clone of v, as such and future changes to v don't effect the clone that was added to data.

Mar 18th, 2008, 2:29 PM	#1
Arla Professional Programmer Join Date: Mar 2005 Posts: 298 Rep Power: 4	Recursion with int arrays So I was playing around with the scrabble problem that was posted, but playing in C# and ran into a quandry, that I'm not 100% sure how to get out of I have a recursion routine to figure out all the possible permutations of an array of int's as follows (Toggle Plain Text) public static ArrayList permute(int[] v,int start, int n) { ArrayList data = new ArrayList(); if (start == n-1) { for (int i=0;i<v.Length;i++) System.Console.Write("{0} ",v[i]); System.Console.WriteLine(); data.Add(v); } else { for (int i= start; i<n; i++) { ArrayList back = new ArrayList(); int tmp = v[i]; v[i]=v[start]; v[start]=tmp; back = permute(v,start+1,n); foreach(object fromPermute in back) data.Add(fromPermute); v[start] = v[i]; v[i]=tmp; } } return data; } public static ArrayList permute(int[] v,int start, int n) { ArrayList data = new ArrayList(); if (start == n-1) { for (int i=0;i<v.Length;i++) System.Console.Write("{0} ",v[i]); System.Console.WriteLine(); data.Add(v); } else { for (int i= start; i<n; i++) { ArrayList back = new ArrayList(); int tmp = v[i]; v[i]=v[start]; v[start]=tmp; back = permute(v,start+1,n); foreach(object fromPermute in back) data.Add(fromPermute); v[start] = v[i]; v[i]=tmp; } } return data; } The problem is that when it does the data.Add(v) it's not really adding v to the data ArrayList, it's just adding a pointer to v to the ArrayList, as such when v gets updated, every pointer to v is pointing to a new version of it, and as such while this returns the right number of records, they are all the same. Is there any easy way I should be getting around this? Playing around with it some I know I CAN get around it but making it a comma delimited string, but that's not exactly elegant.

Mar 18th, 2008, 2:43 PM	#2
Arla Professional Programmer Join Date: Mar 2005 Posts: 298 Rep Power: 4	Re: Recursion with int arrays See and I answered my own question. Changing the line from data.Add(v) to data.Add(v.Clone()) resolved the issue, as the return isn't v, it's a clone of v, as such and future changes to v don't effect the clone that was added to data.

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Programming Forums > Application Development > C#

Recursion with int arrays